The hydrogen-bonded water network in the oxygen-evolving complex of photosystem II

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Abstract

Protein dynamics play a key role in enzyme-catalyzed reactions. Vibrational spectroscopy provides a method to follow these structural changes and thereby describe the reaction coordinate as a function of space and time. A vibrational spectroscopic technique, reaction-induced FTIR spectroscopy, has been applied to the study of the oxygen-evolving complex (OEC) of photosystem II (PSII). In plant photosynthesis, PSII evolves oxygen from the substrate, water, by the accumulation of photo-oxidizing equivalents at the OEC. Molecular oxygen and protons are the products of this reaction, which is responsible for the maintenance of an aerobic atmosphere on earth. The OEC is a Mn4CaO5 cluster with nearby bound chloride ions. Sequentially oxidized states of the OEC are termed the S states. The dark-stable state is S1, and oxygen is released on the transition from S3 to S0. Using short laser flashes, individual S states are generated, allowing vibrational spectroscopy to be used to study these different oxidation states of the OEC. In current X-ray crystal structures, hydrogen bonds to water molecules are predicted to form an extensive network around the Mn4CaO5 cluster. In the OEC, four peptide carbonyl groups are linked to the water network, which extends to two Mn-bound and two Ca-bound water molecules. This dissertation discusses a vibrational spectroscopic method that uses these peptide carbonyl frequencies as reporters of solvatochromic changes in the OEC. This technique provides a new, high-resolution method with which to study water and protein dynamics in PSII and other enzymes.